In 4th Generation, 4G, and 5th generation, 5G, mobile communication networks link adaptation may be used to select an appropriate Modulation and Coding Scheme (MCS). A MCS may be selected based on the available channel information and on a target criterion such as to achieve a given block error rate (BLER) target. In order to select an appropriate MCS, link adaptation relies on measurements provided by the user equipment (UE). These measurements are known as channel quality information (CQI). The base station uses the reported CQI to assign an appropriate MCS to each user. In practice, CQI reports may provide inaccurate or biased information. This may be due to delays in CQI reporting, to missing CQI reports and other reasons. These CQI inaccuracies can cause the network to perform worse than it should and the overall capacity of the network may be affected. The base station also receives acknowledgements that are submitted by a UE after every transmission. These acknowledgements could be either Automatic Repeat Request (ARQ) or Hybrid ARQ (HARQ). In HARQ for example, a HARQ ACK (ACK for short) is reported when the last transmission is correctly decoded and a HARQ NACK (NACK for short) is reported if the UE was not able to decode the last transmission. These HARQ acknowledgments are used by an outer loop in link adaptation to compensate the inaccuracies on the CQI reports. This strategy is known as outer loop link adaptation (OLLA). OLLA is usually implemented as a controller that corrects the current Signal to Interference and Noise Ratio, SINR, estimate, i.e. the controller computes a compensated SINR that is equal to the current SINR estimate plus an outer loop adjustment. The controller for outer loop link adaptation is commonly designed such that an estimate of the BLER based on the HARQ acknowledgments matches a given BLER target.
A classic strategy for outer loop link adaptation to match the BLER estimate with the BLER target is to step up the outer loop adjustment when an ACK is received and to step down the outer loop adjustment when a NACK is received. The ratio between the size of the step up and the step down is chosen to be equal to the ratio between the BLER target and one minus the BLER target. This outer loop link adaptation strategy achieves the desired BLER target.
For OLLA two main features are desired: first OLLA should quickly react to a large change in the SINR inaccuracies. This feature takes great relevance when a UE has just been connected to a base station. When the OLLA response is slow, a recently connected UE may have large transient time in link adaptation that results in an inefficient use of the radio resources. A fast response of the OLLA is also desirable when a large change in the radio channel conditions occurs. These large changes affect the performance of link adaptation mainly because of the delays involved in the CQI reporting. The second desirable feature in OLLA is that it should avoid unnecessary perturbations of the current SINR estimate. The outer loop adjustment should not change when the BLER estimate is close to a BLER target. This reduces the effect of the OLLA over other control mechanisms involved in the communication between a base station and the UE.
Another classic strategy for OLLA is stepping up and stepping down the outer loop adjustment with every HARQ acknowledgement. This strategy has a trade-off between the two desirables features described above. This trade-off is intrinsic in the choice of the step sizes. When the step sizes are chosen to take large values, the outer loop adjustment will quickly compensate for a large change in the inaccuracies of the SINR estimate, but this is at the cost of having large variance around the BLER target. On the other hand, when the step sizes are chosen to take small values, then the variance of the BLER estimate around the BLER target will be small. However, this is at the cost of having a slow response when a large change in the SINR inaccuracies occurs.